Total knee replacement is a common reconstructive procedure. Advances in technique, instrumentation and design over the last 20 years have allowed for improved results with regards to pain relief, return to function and overall quality of life. Over 500,000 patients have knee replacement surgery each year. Total knee arthroplasty surgery typically involves affixing a femoral component to the end of a patient's femur, affixing a tibial component to the top of a patient's tibia, and inserting a tibial insert to serve as a spacer between the two components. The tibial insert has a base that is fixed to the tibial component and, in posterior cruciate substituting designs, a posterior stabilizing post that contacts the femoral component. The femoral component rolls and slides on the tibial insert while the knee flexes until the post contacts a cam on the femoral component. This contact point determines the amount of rollback of the femoral component relative to the tibial component. The human knee and many knee replacements display rollback at high flexion. Rollback is when the femur's contact point with the tibia moves towards the posterior of the tibia. This rollback movement is extremely variable in cruciate retaining designs and in vivo fluoroscopic studies have actually demonstrated paradoxical anterior translation of the femur on the tibia with flexion. One of the advantages of posterior stabilized design is a more predictable pattern of femoral rollback as the knee flexes.
A replacement joint may be customized relative to the patient's anatomy. Preferred replacement joints allow the patient an optimal degree of rollback. Some rollback is often desirable to mimic the behavior of the natural knee, but excessive rollback may be detrimental because it over-stretches the soft tissues surrounding the joint (e.g., the patellar tendon and quadriceps). Over-stretching of these tissues can cause the tissues to tighten, ultimately restricting the patient's movement. Because soft joint tissue structures vary from patient to patient, no single joint replacement may be appropriate for all patients.
In the past, surgeons have tested different trial tibial inserts during total knee arthroplasty surgery in order to identify an appropriate permanent tibial insert. Different thicknesses of trial tibial inserts have been tested in order to identify a suitable insert for a patient. However, currently, the placement of the insert's posterior stabilizing post relative to its base is not selectable at the time of surgery. There is a distinct need for an implant with a variable positioned post. There is also a need for a trial insert with a customizable posterior stabilizing post that a surgeon can evaluate intraoperatively.